1. Field of the Invention
There are many well-known methods for preparing catalysts composed of vanadium-containing compounds deposited upon silicious support materials. Of particular interest herein are methods for preparing catalytic materials composed of one or more oxides of vanadium and one or more sulfates of potassium deposited upon silica support, which catalysts are useful in the oxidation of hydrocarbons to phthalic anhydride.
2. State of the Art
A catalyst for fluid bed oxidation of o-xylene or naphthalene to phthalic anhydride is described in U.S. Pat. No. 3,167,567 to Nonnenmacher et al. The catalyst is prepared by adding vanadium pentoxide to a molten solution of pyrosulfates formed by fusing either potassium pyrosulfate or potassium hydrogen sulfate, or a mixture thereof, then cooling the melt and grinding it to a powder, and thereafter mixing and heating the powder with silica gel to form V.sub.2 O.sub.5 --K.sub.2 S.sub.2 O.sub.7 --Na.sub.2 S.sub.2 O.sub.7 --SiO.sub.2 catalyst. U.S. Pat. No. 3,352,887 to Riley et al. describes a naphthalene oxidation catalyst prepared by forming a melt of vanadium pentoxide and potassium pyrophosphate, which is cooled, ground to a powder and then added to silica gel to form a mixture; heating of the mixture fuses the silica gel and powder. U.S. Pat. No. 3,507,810 to Sanborn et al. describes a naphthalene oxidation catalyst prepared from an aqueous slurry of silica, potassium sulfate and ammonium metavanadate, which slurry is then spray-dried and calcined. A naphthalene-to-phthalic anhydride fluid bed oxidation catalyst, described in Japanese patent publication No. 31630 dated Aug. 23, 1974 of Saito et al., is prepared by mixing a colloidal silica sol with a vanadyl oxalate-potassium sulfate solution to form a slurry, which slurry is subsequently spray-dried and calcined to yield spherical catalyst particles.
A method for preparing a sulfur dioxide-to-sulfur trioxide oxidation catalyst is described in U.S. Pat. No. 4,184,980 to Sherif et al.; the method comprises impregnating a porous diatomaceous-earth support with a potassium sulfate-vanadyl sulfate solution, that solution derived by passing sulfur dioxide gas into an aqueous mixture of vanadium oxide, sulfuric acid and a potassium salt, a typical potassium salt being a sulfate, an oxalate, or a formate of potassium. A similar oxidation catalyst is described in Australian Pat. No. 245,247 to Davies; the catalyst is prepared by incorporating a vanadium compound into a silica gel, with the vanadium compound provided by a soluble vanadyl oxy-salt, such as vanadyl oxalate, vanadyl sulfate, vanadyl acetate and ammonium vanadate.
The foregoing catalysts or preparation methods are characterized by one or more disadvantages, such as requiring multi-step impregnations, utilizing unstable impregnation solutions, or in yielding catalysts that lack uniformity in composition, have low activity, require long conditioning periods, or have high attrition rates.